1. Field of the Invention
This invention relates to an internal shoe drum brake, primarily for motor vehicles, and having a pair of brake shoes mounted on a backplate and arranged for expansion by a mechanism which includes a lever movable angularly in a plane generally parallel to the backplate.
2. Description of the Prior Art
It is common for the drum of such a brake to become grooved by one or both of the shoes after long service and it can become difficult to remove the drum in such circumstances, particularly when the brake includes an automatic adjuster for maintaining the shoe to drum clearance at a predetermined maximum value. In order to overcome this problem, such brakes are often provided with a releasable backstop carried by the backplate, and arranged so as normally to prevent the actuating lever from moving, in the brake release direction, beyond a predetermined retracted position, but, upon release, to permit the lever to move in the brake release direction to a second retracted position, thereby enabling the shoes to move inwardly to clear the drum groove. The drum may then readily be removed for servicing of the brake, for example.
Some examples of releasable backstops are described in our earlier British Patent No. 1247245, these being essentially in the nature of inserts placed in openings in the backplate and capable of manipulation to enable the lever to move from its normal retracted position to the second retracted position, when desired. These arrangements are satisfactory, provided that the backplate profile has sufficient depth to permit the backstop to be placed at a sufficiently steep angle to the plane of the backplate to provide a reliable stop. With some modern shallow backplate designs, these conditions cease to apply and the aforesaid type of backstop becomes unsatisfactory.
In another known arrangement, a resilient shoe hold-down device is provided with a projecting arm which extends to a position behind the plane of the shoe web to provide a backstop for the actuating lever. This can be satisfactory in some light-duty applications, but is not ideal for use in brakes intended for large saloon cars and light vans for example in which considerable shoe hold-down forces are required and hold down devices such as coil springs are then often preferred.